Biology and Epidemiology

- Conservation and sources of inoculum

Golovinomyces cichoracearum var. cichoracearum may survive from one year to another in many ways. The cleistothecia (perithecia without ostiole) that ensure its reproduction may be formed on the oidium infected leaves late in the season. They do not occur very often, but they nevertheless form an overwintering storage. They can be maintained for several years on plant debris of tobacco. Generally, the asci in cleistothecia are formed in autumn, but the ascospores are not ejected until the following spring. The fungus can also overwinter through alternative hosts. The Golovinomyces cichoracearum var. cichoracearum species is polyphagous and is able to attack a large number of hosts, over 200 species belonging to 25 different families. In fact, this species is supposed to contain more or less specialised forms infecting different hosts. Parasitic specialisation of tobacco strains is not well known, they must be able to survive and multiply on secondary hosts, crops or weeds.

- Penetration into the plant and invasion

The infection of the host is carried out from a primary inoculum consisting either of ascospores or conidia. These, once in contact with the host, germinate quickly, likely in two hours if the environmental conditions are appropriate. They form an appressorium and penetrate directly into epidermal cells, developing haustoria. The latter act as sucking organs by extracting nutrient elements necessary for the mycelium growth of the fungus.


- Sporulation and dissemination of the fungus (figures 1 to 3)

Within 4-6 days after infection, short conidiophores born on secondary hyphae formed on the surface of the lamina and produce conidia. Sporulation is abundant. Conidia are quite fragile and live only a few hours or days, when conditions are favorable. They are very light and can be easily carried away and spread by wind and incidentally by rainfall or water splashes during sprinkler irrigation.


- Favourable conditions for the fungus development

Golovinomyces cichoracearum var. cichoracearum seems to be able to multiply at temperatures between 4 and 32° C. Optimum temperature is around 20-25° C. It dislikes the presence of free water on the leaves as well as very high humidity (close to 100 %). It prefers humidity ranging from 60 to 86 %.

The light also influences its development. It appreciates the diffused light, while direct sunlight disturbs its growth. This is certainly the reason why it can be found more on lower leaves, inside the plant canopy, where the surrounding microclimate is particularly mild.

The age of the plants significantly alters the development of the disease. Seedlings are generally less susceptible. In addition, climatic conditions in the nurseries are not always favourable to the fungus. One may consider that in France, in principle, attacks should not be feared in nurseries. After planting and in the following weeks, the plants are still not very susceptible to powdery mildew. But it is no longer the case when the first formed leaves reach their final size, about 6 weeks after transplanting.